The converter having a direct-current intermediate circuit (also referred to as a “Load Commutated Inverter” (LCI)) is a comparatively simple, widely used converter circuit for drive systems. A converter of this type comprises a network-side current converter and a machine-side current converter, the network-side current converter being connected on the input side to an alternating-current network and on the output side to a direct-current intermediate circuit, and the motor-side current converter being connected on the input side to the direct-current intermediate circuit and on the output side to a three-phase drive machine. A choke which serves as an energy store for the intermediate circuit current is usually inserted in the direct-current intermediate circuit, so that, as far as possible, an ideally smoothed, constant intermediate circuit current flows.
A converter of this type is used very frequently for drives with high-power synchronous machines and offers a comparatively simple possibility of IV quadrant operation (i.e. motor and generator operation) of the three-phase drive machine in both directions of rotation.
In motor operation (i.e. when “driving”), the three-phase drive machine draws electrical energy from the alternating current network via the converter. In this case the network-side current converter works in rectifier operation and the machine-side current converter works in inverter operation.
In generator operation (i.e. when “braking”), the three-phase drive machine generates electrical energy and outputs it via the converter to the alternating-current network. In this case the network-side current converter works in inverter operation and the machine-side current converter works in rectifier operation.
The converter having a direct-current intermediate circuit, especially in combination with synchronous machines, is very frequently used for electric ship drives (main propulsion drives). This applies above all to high drive powers in the single-digit or double-digit megawatt range.
In ship drives, especially when propellers with fixed blades (“fixed-pitch propellers”) are used, generator operation in quadrants IV and II is indispensable in order to be able to brake the propeller and to reverse the direction of rotation thereof. The reason is that the moving ship constitutes a “drag-through load” as a result of water flow.
In ship drives having a converter with a direct-current intermediate circuit, the energy released during generator operation is normally fed back into the network via the converter. During a braking process of a ship, this energy can be in the double-digit or triple-digit megajoule range, the braking time being generally within the range from approximately 10 to 60 seconds.
For ships with an onboard network which has only a small so-called “hotel load”, i.e. in which the main electric drive system represents by far the largest electrical consumer, energy recovery to the onboard network is often not desired and/or is not possible with the required power, since in these cases the energy recovery would produce undesired and/or impermissible voltage and/or frequency increases of the onboard voltage. These problems can occur in principle in alternating-current isolated networks, since the number of electrical consumers and therefore the absorption capacity of the network for recovered energy is often limited precisely in isolated networks.
In a solution described in the still unpublished European patent application of the applicant with the official reference number 07022773.1, undesired and/or impermissible voltage and/or frequency increases of the voltage of an alternating-current network of a ship are avoided by converting the recovered energy into heat in a braking resistor which is temporarily connected to the network. Through the braking resistor, therefore, an additional consumer is connected to the onboard network, but the actual energy feedback to the onboard network is not itself prevented. In this case, an abrupt change in the power balance occurs when switching the braking resistor both in and out, placing high demands on the dynamics of the generator control systems.
For drive systems having a converter with a direct voltage intermediate circuit instead of a direct current intermediate circuit, circuits having a braking resistor arranged in the direct voltage intermediate circuit are known. As a rule, such a solution is used when the network-side current converter is in the form of a rectifier with diodes, so that energy feedback to the alternating current network cannot take place. However, because of the essential functional differences between a direct voltage intermediate circuit and a direct current intermediate circuit, such solutions cannot be straightforwardly transferred to a converter having a direct current intermediate circuit.
A ship drive system in which a three-phase drive motor is connected directly to the network without an interposed converter is known from DE 749817. In this case, in generator operation the motor is isolated galvanically from the network and connected to braking resistors by means of a change-over switch.
A ship drive system in which a three-phase drive motor is connected directly to the network without an interposed converter is also known from DE 870 725 A. A motor having a plurality of three-phase and exciter winding systems without reciprocal influences is used. The three-phase winding system used for braking is inactive in motor operation; that is, in motor operation no voltages are induced therein. For braking, a voltage is induced in this three-phase winding system by supplying a suitable exciter current, so that, by means of braking resistors connected to the three-phase winding system concerned, a current which brakes the drive can flow.
Both solutions therefore describe different ways of braking three-phase ship drive motors which are operated directly from the alternating-current network. They cannot therefore be applied straightforwardly to three-phase drives with converter feed, especially those having a current intermediate circuit.